Adult plant resistance genes in wheat: mechanisms, interactions, and breeding strategies for durable rust resistance

Abstract Wheat stem rust, stripe rust and leaf rust pose severe threats to global wheat production. While chemical control becomes increasingly limited by fungicide resistance, genetic resistance remains the most economical and sustainable management strategy. Adult plant resistance (APR) has been widely deployed to protect wheat crops and herein six wheat APR loci that have been cloned and/or extensively used are considered. Four of these loci i.e., Lr34/Yr18/Sr57/Pm38/Ltn1 , Lr67/Yr46/Sr55/Pm46/Ltn3 , Lr46/Yr29/Sr58/Pm39/Ltn2 and Sr2/Yr30/Lr27/Pm70/Pbc1 provide broad-spectrum protection against rust and powdery mildew pathogens while the remaining two, Yr36 and TaEDR2 , provide stripe rust resistance only. Each APR locus confers partial, slow rusting resistance in post seedling plants and is generally insufficient by itself to provide effective levels of disease protection. However, the protection provided by these loci is not race-specific and can also be highly durable, with the Lr34/Yr18/Sr57/Pm38/Ltn1 , Lr46/Yr29/Sr58/Pm39/Ltn2 and Sr2/Yr30/Pm70/Lr27/Pbc1 loci having been widely deployed for many decades without being overcome. In some instances, these APR loci act additively with each other as well as with all-stage resistance genes to confer higher levels of disease protection. The mechanistic basis of APR is not well understood, however causative genes for four of these APR loci have been cloned and their products are diverse ranging from plasma membrane transporters to organelle targeted kinases. The differential additivity of the different APRs suggests they may function by both convergent and nonconvergent resistance pathways. Understanding the combining abilities of these loci is essential for generating the best APR combinations which will be further enhanced by understanding their mechanistic basis to enable accurate combinatorial predictions.